Hypoxia And Ocean Acidification In Two Large Eutrophic Estuaries

Eutrophication-induced hypoxia in the coastal oceans has increased in spatial extent, duration, and severity since at least the 1950s. The sources of organic matter that fuels microbial degradation remain an issue closely related to the policy-making and management strategies. The Pearl River Estuary and the Chesapeake Bay, two of the largest estuaries in the world, both suffer from eutrophication and subsequent hypoxia with different severity under distinct hydrological settings and physical forcing. We conducted field surveys in these two large eutrophic estuaries to reveal the spatial distributions of carbonate system and oxygen, to distinguish the main biogeochemical control, and to quantify the relative contributions of allochthonous (terrestrial) and autochthonous (marine) organic matter to oxygen consumption in the hypoxic zones. Eutrophication can also enhance ocean acidification in the coastal regions. However, less is known about how eutrophic and seasonally hypoxic and anoxic water bodies resist coastal acidification. Based on a spatially-decoupled patterns of removal and addition of Ca2+, TA, and DIC along the main stem of the Chesapeake Bay as well as mineralogical evidence, we reveal that the recovering submerged aquatic vegetation induced by sustained nutrient reduction can serve as an efficient factory to produce CaCO3 solids, which are subsequently transported into the downstream corrosive subsurface waters, and dissolve to buffer pH decrease. This positive feedback to coastal restoration can shed light on eutrophication and acidification studies in coastal systems emerging with recovery signs.